iCloud signon box keeps popping up but it has my old email in gray and willnot let me change it.

I moved about 1 1/2 year ago. I had an email on my iCloud account. After I moved I was given a new email address by a new internet provider. I changed my email address on my iCloud account from my old email to my New email address. Everything has worked fine on my laptop and my phone and iPad. Now all of a sudden I have this box that pops up on my laptop that says sign in to iCloud to continue and ask for a password. But, above the password box it has Apple ID and a gray box with the old email in gray and will not let me change it. There is another larger box that looks like the regular iCloud box and on the left side of that box it has my new email address. When I click cancel on the top box or click on the red X, both boxes go away. I am pretty computer savey and I have tried everything and nothing works to make this box go away. Can someone please help me??